The present invention is directed to thin film composite membranes (TFC membranes) comprising a substrate layer (S) based on a sulfonated polymer, e.g. a sulfonated polyarylether, and a polyamide film layer (F) and further to a method for their preparation. Furthermore, the present invention is directed to osmosis processes, in particular to forward osmosis (FO) processes, using said membrane.
The development and application of membrane technologies (e.g. dialysis, membrane filtration such as nano-, ultra- and micro-filtration and osmosis processes) is one of the most significant recent advances in chemical, environmental, and biological process engineering.
In view of global water scarcity, particularly in drought-prone and environmentally polluted areas, one of the most important applications of osmosis processes using semi-permeable osmosis membrane is the purification of waste water or seawater. Considerable efforts have been put in providing novel methods of purifying wastewater or seawater at lower expenditure with less energy consumption. In this context, membrane based purification and separation processes have become more attractive in comparison to distillation processes.
Generally, the term osmosis describes a diffusion process, wherein solvent molecules move through a selectively permeable membrane (i.e. permeable to solvent but not to solute), which is separating at least two solutions of different solute concentration, into a solution with higher solute concentration. This diffusion process aims for equalization of the solute concentrations. The selectively permeable membrane used in osmosis process is also referred to as “semi-permeable” membrane or osmosis membrane.
Normally, osmosis membranes exhibit a molecular weight cut off (MWCO) in the range from 10 to 500 Da. The molecular weight cut off (MWCO) refers to the lowest molecular weight (given in Daltons) in which al least 80%, preferably at least 90%, of solute molecules are retained by the membrane.
In general, there are two osmosis process modes used in technical applications, namely the reversed osmosis (RO) and the forward osmosis (FO), wherein both osmosis processes utilize a selectively permeable membrane to separate water from dissolved solute molecules or ions.
In the case of reversed osmosis (RO), a hydraulic pressure as the driving force for the separation is employed, wherein the solute retains on the pressurized side of the membrane and the solvent passes through the membrane to the other side. The forward osmosis (FO) employs the osmotic pressure as a driving force generated by a highly concentrated solution (so called “draw solution”) to allow water to diffuse through a semi-permeable membrane from the so called “feed solution” (e.g. brackish water or seawater), which has a relatively lower salt concentration.
Forward osmosis (FO) offers some advantages over the reversed osmosis (RO) and thermal separation processes. For example, forward osmosis (FO) can operate without high hydraulic pressures which are necessary in the reverse osmosis process and high temperatures which are necessary in the distillation and may be detrimental to the feed solution. Less energy is required for the FO process compared to other separation processes. The forward osmosis also offers the advantages of high rejection of a wide range of contaminants and lower membrane-fouling than traditional reversed osmosis processes.
It is known in the state of art that FO membranes can be utilized for example for water reuses, seawater desalination and concentration of pharmaceutical solution. The major problems in use of forward osmosis today are e.g. the limited number of commercially available FO membranes, insufficient water permeation and separation performance of known FO membranes and the lack of desirable draw solutions depended on the intended use of osmosis product.
Membranes, which are designed for reversed osmosis processes can often not be applied in the forward osmosis process due to their thick and dense support layer, which is necessitated to withstand high pressure in the reversed osmosis process and which causes decreased water flux and high salt leakage in the FO process. In this regard, an effective support layer (substrate layer) for FO membranes should be as thin as possible, highly porous, and provide a direct path from the draw solution to the active surface of the membrane.
In state of art several flat several sheet membranes based on cellulose triacetate (CTA) suitable for forward osmosis are known. They are used in applications of water purification for military, emergency relief, and recreational purposes (see T. Y. Cath, A. E. Childress, M. Elimelech, “Forward osmosis: Principles, applications, and recent developments”. J. Membr. Sci. 281 (2006) 70). These cellulose triacetate membranes exhibit a low pure water permeability and salt rejection.
The publication Yip et al. (M. Elimelech, “High Performance thin-film composite forward osmosis membranes”, Environ. Sci. Technol. 44 (3812) 2010) describes thin film composite (TFC) membranes comprising a polysulfone support for forward osmosis application. However, said support layer substrate consists of finger like macrovoid which may reduce the membrane integrity in the long term process of use in osmosis processes, in particular in forward osmosis processes.
The use of polyarylethers and sulfonated polyarylethers for the production of membranes, such as dialysis membranes or membranes in fuel cells, is described in the state of art. Document WO 2009/030620 describes blends of branched polyarylethers and hydrophilic polymers for production of hollow fiber membranes used as dialysis filters. The document WO 2010/142585 describes aromatic polyarylenether block copolymers and their use for the production of polyelectrolyte membranes for fuel cells or for water treatment.
However, there is a high permanent need of novel, superior forward osmosis membrane systems suitable for several applications, which shows high water flux, sufficient salt rejection and excellent chemical and mechanical resistance. Also long-term stability is an important feature.